The present invention relates to a discharge machining control method and a discharge machining control apparatus in which an electrode and a workpiece are located in a machining liquid and machining is executed by applying a voltage between the electrode and the work to generate electrical discharges.
In a discharge machining apparatus for executing machining by applying a voltage to a section between an electrode and a workpiece located in a machining liquid to generate discharges for machining the workpiece, a gap distance control system for adjusting a distance between the electrode and the work is provided for maintaining stable machining conditions. FIG. 11 shows configuration of a machining control system including a gap distance control system based on the conventional technology described, for instance, on pages 88 to 90 in xe2x80x9cMechanism of Discharge Machining and Method for Full Utilization of the Samexe2x80x9d (Gijutsu Hyoron-sha). In this figure, designated at the reference numeral 101 is a discharge machining process, and at 102 a machining state detecting section. Designated at the reference numeral 103 is a reference value setting section, and at 104 an error signal computing section. Designated at the reference numeral 109 is a machining trajectory setting section, at 110 an electrode driving unit. Designated at the reference numeral 111 is a machining pulse condition setting section, at 112 a machining power supply. Designated at the reference numeral 1101 is a controlled variable computing section. Further, y is a value indicating a state of the discharge machining process 101, and ym is a detected value of y by the machining state detecting section 102. r is a reference value for a machining state set in the reference value setting section 103, and e is an error signal obtained from the detection value ym and the reference value r by the error signal computing section 104. Rv is a machining trajectory instruction value set in the machining trajectory setting section 109, and Up is a controlled variable set obtained from the error signal e and the machining trajectory instruction value set Rv by the controlled variable computing section 1101. Mp is an electrode moving quantity for operation according to the controlled variable set Up by the electrode driving unit 110. Rs is a machining pulse condition instruction value set in the machining pulse condition setting section 111. Ms is a machining pulse quantity operated by the machining power supply 112 according to the instruction value set Rs. It should be noted that the machining trajectory instruction value set Rv and controlled variable set Up are vector values corresponding to X, Y, and Z axes and the electrode driving unit 110 is a X, Y, Z-axial driving unit. The gap distance is controlled according to the electrode moving quantity Mp in the X, Y, Z-axial directions. The machining pulse condition instruction value set Rs comprises such parameters as an open voltage, a peak current, a pulse-ON time, and a pulse-OFF time.
FIG. 12 is a view schematically showing the discharge machining process 101. In this figure, designated at the reference numeral 1201 is an electrode, at 1202 a work, at 1203 a machining liquid, at 1204 discharge occurring between the electrode 1201 and the work 1202, and at 1205 a machined surface by means of electric discharges. In the discharge machining apparatus, a gap distance control system as described below is provided for satisfying required machining precision and machining surface roughness and also for optimizing a machining speed.
FIG. 13 is a view showing contents of operations of a gap distance control system based on the conventional technology. Execution of a gap distance control algorithm is generally carried out by means of software processing utilizing a computer, and the k-th time processing is shown in this figure. Step S201 indicates the processing in the machining state detecting section 102, and herein a machining state in a discharge machining process is detected as an average gap voltage ym(k) Step S202 indicates the processing in the error signal computing section 104, and herein an error signal e(k) is computed from a reference value r of the average gap voltage and the detected ym(k). Step S1301 indicates the processing in the controlled variable computing section 1101, and herein a controlled variable set Up(k) is computed from the machining trajectory instruction value set Rv in the machining trajectory setting section 109 and the error signal e(k), and the controlled variable set Up(k) is instructed to the electrode driving unit 110. Kp is a proportion gain, while Ki is an integration gain, and an electrode is controlled so that the detected ym(k) is equalized to the reference value r by means of the technique of PI compensation (proportion+integration compensation).
In recent years, there has been proposed a machining method in which an electrode having a simple form is used and discharge machining having a three-dimensional form is executed by numerically controlling the electrode. Also cases of performing micro machining utilizing the discharge machining are increasing. In the machining method as described above, an area subjected to discharge machining is smaller as compared to that in the conventional technology, and as a result the moving speed of the machining surface becomes higher, so that it becomes difficult to match the detected value to the reference value without any steady state error in the conventional type of gap distance control system. Namely, a state in the discharge machining process is deviated from the optimal state, so that the machining speed becomes disadvantageously lower.
The present invention was made to solve the problems as described above, and it is an object of the present invention to provide a discharge machining control method as well as a discharge machining apparatus in which it is possible to match the detected value to the reference value without any steady state error even when the discharge machining area is small and to improve the machining speed by keeping a discharge machining process in an optimal state.
In the discharge machining control method according to the present invention, an error signal is obtained from a reference value and a value indicating a detected state of machining; a first controlled variable is obtained by adding a value obtained by multiplying the error signal by a proportion gain to a value obtained by multiplying the error signal by a first integration gain for integration; a second controlled variable is obtained by multiplying an instruction value by a second integration gain for integration; and a controlled variable for a driving unit for adjusting a distance between an electrode and a work is obtained by adding the first controlled variable to the second controlled variable and multiplying the sum by a machining trajectory vector.
Further, in the discharge machining control method according to the present invention, the instruction value is previously registered in a data table in correspondence to at least one of the machining pulse conditions, discharge area, or material of the electrode or work, and the instruction value can be changed during machining.
Further, in the discharge machining control method according to the present invention, process parameters are identified with a signal indicative of a position of the electrode or the work and a value indicative of a detected machining state, or with a signal indicative of the speed of the electrode or the work and a value indicative of a detected machining state, and the instruction value is automatically adjusted during machining depending upon the identified process parameters.
Further, in the discharge machining control method according to the present invention, the second integration gain can freely be adjusted by the operator during machining.
Further, in the discharge machining control method according to the present invention, the controlled variable for the driving unit is obtained by adding (i) a vector obtained by multiplying the first controlled variable by the machining trajectory vector to (ii) a vector obtained by multiplying the second controlled variable by the machining trajectory vector and filtering the result with a filter including inverse system characteristics of the driving unit.
The discharge machining control apparatus according to the present invention has a machining power supply unit for generating an electric discharge by applying a voltage to a section between an electrode and a work, a machining pulse condition setting section for setting machining pulse conditions for the machining power supply unit, a machining trajectory setting section for setting a machining trajectory, a reference setting section for setting a reference, a machining state detecting section for detecting a machining state, an error signal computing section for computing an error signal from the reference value and a value indicative of the detected state, a first controlled variable computing section for obtaining a value by adding a value obtained by multiplying the error signal by a proportion gain to a value obtained by multiplying the error signal by a first integration gain as a first controlled variable, an instruction value setting section for setting an instruction value, a second controlled variable computing section for obtaining a value by multiplying the instruction value by a second integration gain for integration as a second controlled variable, and a third controlled variable computing section for adding the first controlled variable to the second controlled variable and multiplying the sum by a trajectory vector set in the machining trajectory setting section to adjust a distance between an electrode and a work.
Further, the discharge machining control apparatus according to the present invention has an instruction value setting section in which the instruction value is previously registered depending upon the machining conditions or the discharge area and the instruction value can be changed during machining.
Further, the discharge machining control apparatus according to the present invention has a process identifying section for computing process parameters from a signal indicating a position of the electrode or the work and a value indicating the detected state of machining, or from a signal indicating the speed of the electrode or the work and a value indicating the detected state of machining, and an automatic instruction value adjusting section capable of automatically adjusting the instruction value during machining depending upon the identified process parameters.
Further, the discharge machining control apparatus according to the present invention has an integration gain adjusting section enabling the operator to adjust the second integration gain during machining.
Further, the discharge machining control apparatus according to the present invention has a third controlled variable computing section for adding (i) a vector obtained by multiplying first controlled variable by the machining trajectory vector to (ii) a vector obtained by multiplying the second controlled variable by the machining trajectory vector and filtering the result with a filter including at least inverse system characteristics of a driving unit for controlling a distance between the electrode and work.